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Imagine a world with time travel, got it? Good.

Now imagine that time has multiple dimensions. Time has its OWN dimension of time, along which it can evolve. This is called hypertime, or "w" (As in w,x,y,z,t).

Now imagine a WH40K style world-government for humanity, that wants history to stay EXACTLY as it is. Time travel is the in-universe equivalent of heresy and any attempts to prevent the presence of our Lord and Emperor are also Treason. When FTL travel is discovered, the TPA is set up to protect history. (This is of course after the Emperor uses it to become Emperor. "After" in the sense of "w" not "t".)

So yeah, laws are made against changing time, and a government agency is set up to make sure there's no time-spirals that could cause instability in the timeline. (Closed Time-Like Curves in relativity look like spirals when you add in the hypertime dimension.)

They can't move or detect ANYTHING that is in the "hyperfuture". They can use probes or whatever to detect anything in spacetime, but not hypertime. Hypertime passes when the events IN time are changed, so as history changes, hypertime progresses. Time is where space evolves, hypertime is where spacetime events evolve and it only makes sense to think of hypertime passing as events in history are changed.

The Questions are:

How do you detect time travel?

What is the simplest way to have a time agency keep a record of the "TRUE" history.

What is the least handwavium way to have them detect that they've progressed in hypertime?

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  • $\begingroup$ can't jump also means can't look, right? $\endgroup$ – Mołot Mar 9 '17 at 0:38
  • $\begingroup$ Assuming hypertime passes for all time travelers at the same rate, they can look about in TIME, just not hypertime. So if they look into the past they'll see someone killing Hitler, but they won't see the succesful mission to take her down that happens several hours, for them, later. Because it's later in hypertime. $\endgroup$ – Douglas Mar 9 '17 at 0:44
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    $\begingroup$ Could you explain your time travel model? The link you provide does not match your scenario. There are either one (no detection possible) or infinite actualized timelines (detection trivial but pointless since somebody actualizing a new timeline has zero impact on you, why would you care that all timelines uptime now have nazis winning WW2, they can't do anything to you and actualizing a new timeline with allied victory would do nothing to them). $\endgroup$ – Ville Niemi Mar 9 '17 at 2:17
  • $\begingroup$ WH40K style? Just make time travel heresy. No follower of the Emperor would commit heresy. $\endgroup$ – kathyra Mar 9 '17 at 5:32
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    $\begingroup$ The essential problem with Time Police stories is: How can there be a concept of The "Right" Sequence of History? Why is a 'wrong' event wrong? Who decides? With what criteria? And why are they 'right' and not somebody else? Until that is answered, nobody can even begin to address your question. $\endgroup$ – Spencer Mar 9 '17 at 11:06
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Create as complete of a record of the past as you can. Make two copies of it on read only media.

Store the backup copy either in a dimension in a separate timeline or, if that isn't possible, in the very far past of your timeline. Then leave a portal open that allows communication. Have the backup copy, in a continuous loop, transmit its data through the portal. Have no communication to the backup location. Destroy any records of the backup location and kill anyone who knows where it is.

Use that to check your copy against the backup copy.

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  • $\begingroup$ This is great, thanks. I can make a device containing all of known history. Put it in a bootstrap paradox in the far past and then it would go round in a spiral. Falling through hypertime but the information on it always coming from the same, former, version of the timeline. Then compare that to the copy in the TSA database computer, as you suggest, and that's how they find changes in the past. $\endgroup$ – Douglas Mar 9 '17 at 21:49
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There is a theory/hypothesis about time. Namely that all time has already happened. Basically, time, like any other dimension, is, relatively, static. We as humans are not capable of perceiving all of time, just like we are incapable of perceiving all of space.

However, we do have the technology to monitor all of space (though some limits apply and your results may vary). So what your TPA needs is to send a satellite into outer time. Or in more official terminology they need a detector that can monitor time (t) and hypertime (w). Since the TPA is aware of w, it means they can observe w. They can measure w and its interactions with regards to t.

Once you have such a sensor, you just have to calibrate it and produce multiple of them. You calibrate the sensor with theoretical values adjusted for the background noise in w.

A semi decent example is LIGO, it detects the bending of the fabric of space time due to gravitational waves based on knowledge of the speed of light, among other things. Another is GPS. GPS works on our knowledge of gravity's effect on time, among other things.

That said: it will require that your world has a sound theory of the general relativity between w and t, because that's how your sensor is going to have to work. But since the TPA can observe w and draw relationships between w and t, this really shouldn't be that hard.

The easy part is after that. They detect an anomaly as based on their measurements, narrow it down to its XYZT coordinates, using Hyper General Relativity, and viola. You got your perp.

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If the TPA is so vehemently opposed to time travel that they aren't willing to even do it themselves to keep others from doing it, I believe it's impossible without some way of intercepting time travelers in $t$ before they go back in time. If they can look forward in $t$, and depending on the details of your scenario, this may be possible.

However, if they are willing to go back in time themselves ("for the greater good"), and there is no such thing as the butterfly effect, and time criminals are altering time in order to change historic events, there's a simple solution: the TPA can place a secret base of operations in prehistoric times. This way, they can periodically send agents (or look forward in $t$, again, if that is possible) to check the historical timeline for meddling, similar to @DJClayworth's answer. However, now when time is meddled with, the TPA don't have to worry about their base being erased from existence, because the base's $t$ is before the alteration's $t$, and they can merely intercept the offending time traveler when in $t$ they arrive and detain them (probably permanently, so that no one can reveal their secret). There's plenty of opportunity for interesting plot complications with this:

  • Someone accidentally discovers the existence of the secret base and is pursued by the TPA through hypertime.
  • Someone wants to alter history, but knows about the secret and mounts an attack on the prehistoric base in order to disable it first (in $w$).
  • etc.

Of course, if a time traveler goes back to prehistoric times and changes something, either on purpose or accidentally (à la A Sound of Thunder), nobody can do anything about it, and depending on the details of your scenario, the base might get erased from existence. Equally of course, most of this is invalid if the TPA won't use time travel themselves.

EDIT: As @JoshKing so kindly pointed out, this is very similar if not identical to Poul Anderson's Time Patrol. This probably makes the above more likely to be valid but less interesting in any case.

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    $\begingroup$ Congratulations you just invented Poul Anderson's Time Patrol! $\endgroup$ – Josh King Mar 9 '17 at 5:05
  • $\begingroup$ @JoshKing Oops, I hadn't heard of that. I'll make an edit to point that out. $\endgroup$ – Aidan F. Pierce Mar 9 '17 at 5:08
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I think you need to firmly decide what happens to the present when an event changes Hypertime. There are a few possibilities:

  1. The change overwrites the current "time." The changes are part of history and always have been. This would corrupt/overwrite whatever checking mechanism you can device that is within the same timeline. The plus side is that the Empire can honestly say that they have not detected any corruption to the timeline.

  2. The change splits off a parallel universe. People of Universe A maintain their original timeline, but Universe B has the new timeline. These sorts of changes could be detected, and, assuming that you can cross into a new parallel dimension, could be "eradicated" by destroying the parallel universe. The original timeline would remain safe (though, in theory, if there was a Universe B version of the Temporal Inquisition, they would believe they were the originators and try to destroy Universe A in a similar way).

  3. The change causes a "shockwave" that slowly changes the current Timeline to the new Timeline. This could be detectable by scanning historic events and comparing them to a database. It will also create dramatic tension, as once the "shockwave" reaches the current timeline, it will overwrite the current timeline.

My personal favorite is 2, as it can be the basis for time wars. By the time the story occurs, there could be infinite parrallels, with war raging between many of them.

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Every so often in w, you check key events.

It's assumed that if you can jump to a specific point in t you can also investigate events in t (possibly by jumping there very very discreetly), and check that they are happening like you expect. Night shift guy at the TPA has to do this.

Routine check of 1946. Wait, Hitler not dead? Press the big red alarm button!

In answer to comments, I am assuming changes to the timeline don't affect people in W. Night shift guy then has a simple check list based on the expected state of things in 1946. 1. Hitler dead 2. Atomic bomb dropped 3 Montreal Canadiens won Stanley Cup. The list isn't affected by changes in the timeline (If they were, how would anyone ever know the timeline had been changed).

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  • $\begingroup$ How would you even know to press the big red alarm button? I would think you need a second person to jump while the event is changed. Otherwise, when you get there, if history changed, your knowledge, since you are no longer in the same frame of reference as when you left, becomes your new fact and you don't know any better. Yet a second person jumping is still in the same frame of reference and when they arrive, will know something changed. (Think of frame of reference in space travel. The way twins age differently, and when in Warp, your frame of reference stays in sync with where you left) $\endgroup$ – George McGinn Mar 9 '17 at 1:51
  • $\begingroup$ DJ, the thing is EVERYONE is in both w and t. Changing events in xyzt is what causes movement along the w axis. The idea is that time travellers have a kind of "San Deemas Time" clock that means causality still works, just not in an obvious way. The problem with your idea is that I want them to be able to find anyone in time that shouldn't be there. Going back in time to check every key event means eventually you'll have an infinite amount of time travellers checking in on Hitler to make sure he's tucked in. To say nothing of just how many events in history they'd have to check every night. $\endgroup$ – Douglas Mar 9 '17 at 12:50
  • $\begingroup$ So basically, their job is to read up on history for anyone who shouldn't be there, then go back and prevent them from being there. $\endgroup$ – Douglas Mar 9 '17 at 12:52
  • $\begingroup$ This is basically the plot of the first Journeyman Project game. $\endgroup$ – TheBlackCat Mar 9 '17 at 15:44
  • $\begingroup$ See also "The End of Eternity" $\endgroup$ – DJClayworth Mar 9 '17 at 15:53

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